Confinement of Wastes Injected below Thawed Permafrost-- A 12 Year Update from the North Slope of Alaska

WEINGARTEN, JEAN, ARCO Inc.

The main North Slope Class I industrial waste disposal facility has injected 12 million barrels into a permeable formation beneath the permafrost at 2000 feet. Initial concerns about fluid confinement were addressed by extensive field testing. This testing, coupled with no underground sources of drinking water, allowed injection pressures to be authorized above the fracture gradient; an unusual operating condition.Re-permitting the facility in 1999 required an evaluation of permafrost thermal response to the injection of warm fluids and determination of potential impacts on waste confinement. This was accomplished using temperature logging after an extended shut-in period and thermal modeling. At issue was whether the temperature log profiles could be explained by conductive heat transfer or whether they indicated upward fluid movement out of the injection zone. This required coupling a radial thermal model with a one dimensional vertical conduction model. This combination could thus predict the temperature profile adjacent to the injection interval because vertical heat transfer is the dominant thermal mechanism just above the storage reservoir.Very good agreement was obtained between field measurements and the model results, indicating there was no fluid moving upward through the confining zone. With this "history match" as a basis, the model was used to predict future temperature changes and thaw bulb growth throughout the permafrost interval.The paper addresses concerns about wellbore channeling, confining zone integrity, and over-all waste confinement while injecting above the fracture gradient adjacent to the base of thawed permafrost. This work has been instrumental in understanding what is occurring in the subsurface.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California